7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Magnetic reconnection: Theory and modelling for space and astrophysicalplasmas
D.I. Pontin1, E.R. Priest2 and L.P. Chitta3
1
School of Information and Physical Sciences, University of Newcastle, Australia
2
School of Mathematics and Statistics, University of St Andrews, UK3
Max Planck Institute for Solar System Research, Göttingen, Germany
e-mail (speaker): david.pontin@newcastle.edu.au
Magnetic reconnection is a fundamental astrophysicalprocess: it plays a key role in the dynamics of the Sun's
atmosphere and planetary magnetospheres, as well asother astrophysical objects. It facilitates the rapid
conversion of stored magnetic energy into other forms
such as kinetic and thermal energy. This drives
phenomena such as flares, eruptions and jets in the Sun,stars, and beyond, as well as contributing to coronalheating. In the magnetosphere it is involved in flux
transfer events and substorms. This talk will begin with areview of the fundamental theory of three-dimensional
magnetic reconnection [e.g. 1].
In a highly-conducting plasma the magnetic topology ispreserved as the plasma evolves, an idea encapsulated by
Alfvén’s frozen flux theorem [2]. Magnetic topology
conservation therefore means that magnetic field lines
cannot break or merge with one another, but instead
evolve via smooth deformations. Here “magnetictopology” refers to the connectivity and mutual linkage
of magnetic field lines within the domain of interest, as
well as field line connectivity between points on thedomain boundary. Real plasmas have finite conductivity
with the result that the magnetic topology is notpreserved everywhere for all time: field lines break andrecombine in small sub-domains by the process ofmagnetic reconnection, permitting a reconfiguration of
the magnetic field and often a rapid conversion of stored
magnetic energy to other forms. A significant challenge
for our understanding of reconnection is how thedynamics on global length scales are coupled with themicro-scale (kinetic) processes that take place in the(a) Magnetically closed

dissipation region within which the reconnection takes
place.
Recent observations from the Solar Orbiter and Parker
Solar Probe missions have shed new light on theimportant role of small-scale magnetic reconnection(involving flux domains on the scale of granules) incoronal heating and in the generation of the solar wind[e.g. 3]. In the second part of this talk I will briefly
introduce a new model [4] that suggests that interchange
magnetic reconnection induced by photospheric flux
cancellation may play a leading role. I will show thatthrough two phases of atmospheric energy release -- precancellation and cancellation -- the cancellation ofphotospheric magnetic flux fragments may provide asignificant contribution to coronal heating and solar wind
generation, depending on the global field geometry (seeFigure 1).
References[1] D.I. Pontin and E.R. Priest, Magnetic reconnection:
MHD theory and modelling, Living Reviews in
Solar Physics, 19 (2022).
[2] H. Alfvén, On the Existence of ElectromagneticHydrodynamic Waves. Arkiv för matematik,astronomi och fysik. 29B(2): 1–7 (1942).
[3] SD Bale et al., Interchange reconnection as thesource of the fast solar wind within coronal holes,
Nature 618, 252–256 (2023)[4] D.I. Pontin, E.R. Priest, L.P. Chitta and V.S. Titov,Coronal Heating and Solar Wind Generation by
Flux Cancellation Reconnection, in preparation(b) Magnetically open******+++++**- - - - - ++ - - -

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Figure 1. From [4], a unified model for coronal heating and solar wind acceleration in regions above supergranules thatare (a) magnetically closed and (b) magnetically open. Stars indicate reconnection sites
in the lower atmosphere thatnulls
generate flows (red arrows) as well as fast particles, waves and heat flux (blue arrows).